This invention relates to a cold cathode fluorescent lamp, a cold cathode fluorescent lamp driving apparatus, a cold cathode fluorescent lamp apparatus, a liquid crystal display apparatus, a control method for a cold cathode fluorescent lamp, and a control method for a liquid crystal display apparatus.
Known fluorescent lamps are roughly classified into those which are driven by ac current and those which are driven by dc current. Further, fluorescent lamps can be classified also into those of the hot cathode type and those of the cold cathode type. Also those fluorescent lamps are known which light up, at an initial stage of lighting, as those of the hot cathode type and thereafter operate as those of the cold cathode type, as disclosed, for example, in Japanese Patent Laid-Open No. 2000-294391 (hereinafter referred to as Patent Document 1).
While such fluorescent lamps as mentioned above are industrially used widely, attention is paid in recent years to applications of a fluorescent lamp to a cold cathode fluorescent lamp apparatus (backlight apparatus) for irradiating light from the back of a panel of a liquid crystal display apparatus. As a light source for use with a backlight apparatus of a liquid crystal display apparatus, attention is paid particularly to a cold cathode fluorescent lamp (CCFL) which uses a cold cathode which need not be heated and has a comparatively long service life.
A cold cathode fluorescent lamp for use with a backlight apparatus is normally driven by a high voltage of a high oscillation frequency of approximately 30 to 50 kHz in order to suppress flickering caused by use of ac current. Further, as a circuit for driving a cold cathode fluorescent lamp, an inverter circuit is widely used which converts commercial dc power supply into ac high frequency power supply, which is supplied to the cold cathode fluorescent lamp.
Further, together with increase of the display area of a liquid crystal display apparatus in recent years, also a cold cathode fluorescent lamp for use with a backlight apparatus which irradiates light from the rear face of a liquid crystal panel is progressively inclined to be provided with an increased length conforming to the expansion of the display area of the liquid crystal panel. Further, together with the increase of the display area of the liquid crystal display apparatus, it is common to adopt a direct backlight apparatus wherein a plurality of cold cathode fluorescent lamps are juxtaposed in parallel to each other to form a planar light source and disposed on the rear face of the liquid crystal panel to irradiate light upon the liquid crystal panel. In a backlight apparatus of the type described, a reflecting member is provided on the rear face of the cold cathode fluorescent lamps to raise the utilization efficiency of light or an optical sheet such as a diffusing sheet or a prism sheet is interposed between the backlight apparatus and the liquid crystal panel to enhance the luminance. In this manner, various optical structures have been devised for the backlight apparatus, and a structure which surrounds a cold cathode fluorescent lamp as a light source is adopted.
Further, as a driving circuit for a cold cathode fluorescent lamp, also such an improved collector resonance circuit as shown in FIGS. 42 and 43 is used, as disclosed, for example, in Japanese Patent No. 3,230,540 (hereinafter referred to as Patent Document 2).
Such liquid crystal display apparatus as described above have a problem in that driving power of a high frequency applied to a cold cathode fluorescent lamp flows as leak current through a floating capacitance formed between the cold cathode fluorescent lamp and various optical members provided around the cold cathode fluorescent lamp such as a reflector and a diffusing plate. The magnitude of the floating capacitance increases from increase of the length of the cold cathode fluorescent lamp by expansion of the display area, adoption of a plurality of cold cathode fluorescent lamps juxtaposed in parallel to each other and frequent use of various optical members. The increase of the magnitude of the floating capacitance progressively increases the magnitude of the leak current and makes it difficult to effectively utilize the driving power.
Further, in a cold cathode fluorescent lamp having an increased length, leak current appears intermediately in the longitudinal direction and gives rise to variation of the light emission amount at different portions in the longitudinal direction. In particular, the current flowing through the inside of the cold cathode fluorescent lamp increases toward the electrodes but decreases away from the electrodes. Therefore, the cold cathode fluorescent lamp exhibits such difference in brightness that the brightness increases toward the electrodes but decreases away from the electrodes. This phenomenon becomes notable as the elongation of the cold cathode fluorescent lamp increases.
Therefore, in order to decrease high frequency leak current where ac driving wherein a high voltage of a high frequency is applied to a cold cathode fluorescent lamp is adopted, it is a possible countermeasure to increase the spatial distance between the cold cathode fluorescent lamp and surrounding structures to decrease the floating capacitance. However, this countermeasure involves increase in thickness of the structure of the backlight apparatus and makes it difficult to reduce the thickness of the liquid crystal display apparatus.
Thus, it is desirable to provide a liquid crystal display apparatus which solves the above-described problems involved in ac driving and a cold cathode fluorescent lamp, a cold cathode fluorescent lamp driving apparatus, a cold cathode fluorescent lamp apparatus, a control method for a cold cathode fluorescent lamp, and a control method for a liquid crystal display apparatus, which are suitable for use with the liquid crystal display apparatus.